Preservation of organic molecules in silica and sepiolite clay under UV radiation

Abstract

On Earth, alteration minerals, such as clays and amorphous silica, have proved to be able to preserve organic molecules linked to life over geological timescales. These minerals are also widespread on the Martian surface and have been shown to be associated with organic molecules by the rovers. However, continuous bombardment by radiation of the surface, including UV, raises questions about the alteration state of the organic molecules and, consequently, the preservation capacity of the mineral phases that host them. To investigate this, we experimentally exposed natural organo-mineral complexes to UV radiation, comprising four mineral matrices (opal, chalcedony, sepiolite and silicified sepiolite) naturally coloured in pink by chromophoric polycyclic aromatic hydrocarbons known to be photodegradable. We monitored the evolution of the intensity of the pink colour through UV–visible spectroscopy as a proxy of the degradation of the organic molecules. Results show that all the mineral matrices possess a distinct preservation capacity of their pink colour for which opal and silicified sepiolite (channels filled with amorphous silica) proved to be the most effective. Moreover, our results tend to indicate that less than an equivalent of 100 years of UV-exposure at the surface of Mars are sufficient to drastically alter the organic molecules, even embedded in a mineral matrix. We infer that to maximize the chances of detecting the best-preserved organic molecules on the surface of Mars, priority should be given to silica-rich alteration phases and silicification events as key astrobiological targets.